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IT 601: Mobile Computing
Wireless Sensor Network
Prof. Anirudha SahooIIT Bombay
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Wireless Sensor Networks
• How is it different from traditional wireless network?– for specific application– embedded system with very limited resources
(memory, battery, os)– typical deployment with thousands of nodes– data-centric, individual node’s performance not
important
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Applications for WSN
• Environment and habitat monitoring• Precision Agriculture• Indoor climate control• Military surveillance• Intruder detection• Earthquake/volcano prediction• Patient vitals monitoring
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WSN System Challenges• Very Large Scale
– Dense instrumentation– Limited device capability
• Sometimes partial measurements have to be correlated– Limited Access
• Deployed in remote places• Leverage wireless communication to gather information
– Dynamic condition• Environmental condition, event reporting (change in load)• Death of nodes
– Change in topology– Routing protocol
– Operating Systems• Should be scaled down to fit the embedded architecture
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WSN OSs
• Should be scaled down to fit the embedded architecture
• Example OSs– VXWorks, Linux variants, WindowsCE, GeoWorks
• Component based OS– TinyOS from Berekeley
• Concurrency, fine-grained power management, light-weight event scheduler
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MANET vs WSN
Coordinated effort among nodes, data-
centric, mostly low data rate
Individual nodes
important, ID centric, high
data rates
Communication
largeSmallScale
embedded system (with constrained resources)
More powerful, relatively more
resources
Devices (energy)
SpecificGeneralApplication
WSNMANET
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WSN MAC
• Attributes– Collision avoidance– Energy efficiency ------ important– Scalability and adaptability
– Channel utilization– Latency– Throughput ----------- not so important– fairness
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Classification of MAC
• Scheduled protocols– Nodes send data in predetermined times
• TDMA, FDMA, CDMA– Contention based protocols
• Nodes compete with probabilistic coordination
– ALOHA, CSMA
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Scheduled vs contention-based protocols
Loose or not needed
preciseTime sync
easyDifficultMultihop communicati
on
goodbadscalability
badGoodEnergy efficiency
yesNocollision
Contention-based
protocols
Scheduled protocols
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Energy Efficiency in MAC
• Sources of Energy Wastes– Collision– Control packet overhead– Overhearing unnecessary traffic– Idle listening (a major source of energy waste,
consumes 50-100% of the power for receiving)
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S-MAC
• Major Features of S-MAC– Collision avoidance– Periodic listen and sleep– Overhearing avoidance– Adaptive listening– Message passing
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Collision Avoidance
• Based on CSMA• Similar to 802.11 DCF
– Physical and virtual carrier sense– Randomized backoff– RTS/CTS for hidden node problem– RTS/CTS/DATA/ACK sequence
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Periodic listen and sleep
• Since idle listening consumes lots of energy, S-MAC employs periodic listen and sleep
• Turn off radio while sleeping• Reduce duty cycle to ~10%• Increases latency but decreases energy
consumption
listen listensleep
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Periodic listen and sleep
• Neighboring nodes will have the same schedule.• But two nodes who are multihops away, may end up
with different schedules• Border nodes will follow two schedules
– This enables broadcast packets to be sent only once across the two clusters
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Overhearing avoidance
• Problem– Nodes receive packets destined for others
• Solution– Sleep when neighbors talk
• Who should sleep?– All immediate neighbors of sender and receiver
• How long to sleep?– The duration field in each packets should be the
duration that the neighbors should sleep
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Adaptive Listening
• Reduces latency in multi-hop scenario• Wake up for a short period of time after transmission
from neighbors (if it overheard the corresponding RTS/CTS)– This way, if the node is the next hop node, the node
will be able to send the data immediately instead of waiting for the scheduled listen time
– Reduces latency, but increases duty cycle => more energy consumption
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Message Passing• Fragment large messages into small fragments• Have one RTS-CTS exchange for the entire message
– Reserve the medium for the entire message• But ACK is sent by the receiver for every fragment
– If an ACK is not received, only that fragment is retransmitted and the reservation period is extended for one more fragment
• If the entire msg were sent at once, then retransmission would have been costlier
• If only one fragments were sent per RTS-CTS, the control overhead would have been higher and the msg level latency would have been higher
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Implementation• Platform
– Mica Motes (UC Berkeley)• 8-bit CPU at 4MHz,• 128KB flash, 4KB RAM• 20Kbps radio at 433MHz
– TinyOS: event-driven (modified stack)• Configurable S-MAC options
– Low duty cycle with adaptive listen– Low duty cycle without adaptive listen– Fully active mode (no periodic sleeping)
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Experiment
Ten hop linear topology
Source: S-MAC paper
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Energy Consumption vs. msg inter-arrival
Source: S-MAC paper
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Latency vs number of hops
Source: S-MAC paper
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References
• W. Ye, J. Heidemann and D. Estrin, “Medium Access Control with Coordinated Adaptive Sleeping for Wireless Sensor Networks”- IEEE Transactions on Networking, vol. 12, No. 3, June 2004.